Multi-component adsorption isotherm is determined experimentally when He, H2 (or D2) and CH4 are adsorbed on activated carbon (AC) plated on a cryopanel cooled at cryogenic temperature and desorbed at room one. It is correlated to the Langmuir-Freundlich-type one. The equilibrium isotherms for their respective single-component adsorption processes are correlated in terms of the Langmuir-Freundlich-type ones, and the isotherm when the three components of He, H2 and CH4 are adsorbed at the same time is correlated in terms of a naturally-extended multi-component one without any changes in the original constants included in the single-component one. Rates of the isotopic exchange reaction of D atom between CH4 and D2 on AC between 10 K and room temperature are also determined. After eliminating the effects of natural isotope of 13C included in CH3D, CH2D2, CHD3, CD4 on mass-spectrometric measurements, it was found that the isotopic exchange rate of CH4 + D2 = CH3D +HD and so on was found to be so late that any detectable effect was not observed.